Conventionally, when it is desirable to have a solid propellent rocket engine with a steerable ejection nozzle, the nozzle is mounted on a flexible hinge whose fixed portion is secured to the wall of the combustion chamber of the engine. To facilitate such mounting and to enable the nozzle to be integrated in the chamber, the rear end of the nozzle generally has a special shape like the "kick" or "punt" at the bottom of a bottle, and unfortunately, by defining an annular trough between the wall of the nozzle and the wall of the chamber, that shape creates a trap in which solid or liquid bodies can collect during the flight of the thruster.
When solid propellent burns, it causes combustion residues to be formed. For solid propellents that contain aluminum, one of the residues is alumina. During combustion of the propellent (which takes place at temperatures higher than the melting point of alumina), said alumina is in the liquid state, in the form of fine droplets. These droplets are of a size that depends essentially on the grain size of the propellent (in the range 20 .mu.m to 120 .mu.m, for example), and they are normally entrained by the flow of propulsion gases simultaneously with the other combustion products, and they solidify during expansion prior to being evacuated in solid form through the nozzle.
However, the greater the size of such droplets, the more difficult it becomes to entrain them towards the nozzle by means of the flow of propellent gases. This applies in particular for droplets having a size of about 100 .mu.m which, instead of being evacuated through the nozzle, collect in the above-mentioned trap where they end up by forming a puddle of liquid alumina. Given the masses involved, this puddle then behaves like ballast which decreases the payload that can be put into orbit in a manner that is particularly significant, given that 250 tons of propellent can produce a residual mass of 2 tons of alumina.